Kleisli endo-morphisms are an instance of Monoid.

main.hs

{-# LANGUAGE FlexibleInstances, OverlappingInstances #-}
module Main where
import Data.Monoid
import Control.Monad

instance Monad m => Monoid (a -> m a) where
    mempty  = return
    mappend = (>=>)

main :: IO ()
main = do
  let m  = \x -> [x * 2, x * 3]
  let m' = \x -> [x + 1, x, x - 1]
  let mm' = m `mappend` mempty `mappend` m'
  print $ mm' 2